1. Field of the Invention
The present invention relates to a piezoelectric transformer that can be used in various high voltage generating devices, and in particular to a piezoelectric transformer used in an inverter circuit for carrying out the light emission control of a cold cathode tube.
2. Description of Related Art
FIG. 12A is an outside perspective view showing the structure of a Rosen-type piezoelectric transformer, which is a typical structure for conventional piezoelectric transformers. FIG. 12B is a cross-sectional view in the length direction of the piezoelectric transformer shown in FIG. 12A. The advantages of this piezoelectric transformer are that it is smaller than magnetic transformers, that it is nonflammable, and that it does not generate noise due to magnetic induction, for example.
The portion denoted by the numeral 2 is the low impedance portion of the piezoelectric transformer, which serves as the input portion when the piezoelectric transformer is used to step up voltage. The low impedance portion 2 is polarized in the thickness direction (indicated by PD in FIGS. 12A and 12B) of a piezoelectric ceramic plate 1 (hereinafter, referred to as piezoelectric plate), an electrode 3U is disposed on the first principal face, and an electrode 3D is disposed on the second principal face in the thickness direction. On the other hand, the portion denoted by the numeral 3 is the high impedance portion, which serves as the output portion when the piezoelectric transformer is used to step up voltage. The high impedance portion 3 is polarized in the length direction (indicated by PL in FIGS. 12A and 12B) of the piezoelectric plate 1, and an electrode 4 is disposed on the end face of the length direction. In piezoelectric transformers configured in this way, the transverse effect of the piezoelectric longitudinal vibration is utilized in the low impedance portion 2 and the longitudinal effect of the piezoelectric longitudinal vibration is utilized in the high impedance portion 3.
The operation of a conventional piezoelectric transformer with the above configuration is described next using FIG. 13.
FIG. 13 is an equivalent circuit diagram approximated with the concentrated constants near the resonance frequency of the piezoelectric transformer in FIGS. 12A and 12B. In FIG. 13, Cd1 and Cd2 are the bound capacitors on the input side and the output side, respectively, A1 (input side) and A2 (output side) are the force factors, m is the equivalent mass, C is the equivalent compliance, and Rm is the equivalent mechanical resistance. In the piezoelectric transformer shown in FIGS. 12A and 12B, the force factor A1 is larger than A2, and the voltage is stepped up by two equivalent ideal transformers 1301 and 1302 in FIG. 13. The piezoelectric transformer includes a series resonant circuit made of the equivalent mass m and the equivalent compliance C, and thus the output voltage has a value equal to or greater than the transformation ratio of the transformer, in particular when the value of the load resistance is large.
Besides this Rosen-type piezoelectric transformer, another piezoelectric transformer known in the art utilizes the radially expanding vibration of a disk, as shown in FIGS. 14A and 14B. FIG. 14A is a front view of this piezoelectric transformer and FIG. 14B is a cross-sectional view taken along the line A–A′ in FIG. 14A. In FIG. 14B, the low impedance portion is denoted by the numeral 224 and the high impedance portions are denoted by the numerals 223 and 225. The low impedance portion 224 is made of electrodes 221U and 221D, and the high impedance portions 223 and 225 are made of an electrode 222 and the electrode 221U or 221D. As in the case of the Rosen-type piezoelectric transformer shown in FIGS. 12A and 12B, the low impedance portion 224 utilizes the transverse effect and the high impedance portions 223 and 225 utilize the longitudinal effect.
A piezoelectric transformer with this structure is represented by the equivalent circuit of FIG. 13, like the Rosen-type piezoelectric transformer of FIGS. 12A and 12B, and also operates in the same way.
In a piezoelectric transformer like that shown in FIGS. 14A and 14B, however, the low impedance portion 224 utilizing the transverse effect must be polarized in the thickness direction of the disk and the high impedance portions 223 and 225 utilizing the longitudinal effect must be polarized in the radial direction of the disk. This causes extremely intense stress on the piezoelectric transformer during the poling process, which increases the risk of cracks or the like occurring near the boundary between the low impedance portion 224 and the high impedance portions 223 and 225.
To solve this problem, JP 2666562B proposes a piezoelectric transformer that employs the radially expanding vibration of a disk and utilizes the transverse effect for both the low impedance portions and the high impedance portions. The structure of the piezoelectric transformer disclosed in JP 2666562B is shown in FIGS. 15A and 15B. FIG. 15A is a front view of this piezoelectric transformer and FIG. 15B is a cross-sectional view taken along the line A–A′ in FIG. 15A.
However, since the piezoelectric transformer shown in FIGS. 15A and 15B employs the transverse effect for the low impedance portions 215 and 217 and the high impedance portion 216 alike, the capacitance of the output portion of the piezoelectric transformer becomes larger than those of the Rosen-type piezoelectric transformer shown in FIGS. 12A and 12B and the piezoelectric transformer shown in FIGS. 14A and 14B, which utilizes the radially expanding vibration of a disk.
It is well known in the art that when a cold cathode tube is used as the load for the piezoelectric transformer, the piezoelectric transformer can be driven efficiently by designing the output impedance of the piezoelectric transformer and the load impedance of the cold cathode tube to be substantially equal. However, using the transverse effect for the output portion makes the output impedance of the piezoelectric transformer much smaller than the load impedance of the cold cathode tube, which causes a drop in efficiency when driving.
One approach to solving this problem is proposed in JP 2000-49399A, which discloses the structure of a piezoelectric transformer employing a piezoelectric plate that has a single polarization direction. FIG. 16 shows the structure of the piezoelectric transformer proposed in JP 2000-49399A. In FIG. 16, the numerals 201 and 204 denote electrodes constituting the high impedance portion and the numerals 202 and 204 denote electrodes constituting the low impedance portion. Here, the capacitance of the output portion of the piezoelectric transformer can be controlled by forming a small electrode 201 on the principal face in the thickness direction of a piezoelectric body 203. However, making the electrode 201 small increases the region that does not contribute to the propagation of energy, therefore lowering the coupling factor resulting from the electrode structure and the vibration mode (hereinafter, referred to as the effective coupling factor). This is problematic because it increases loss, lowers element efficiency, increases the amount of generated heat, and also increases stress.